On Oct 8, 9:36*am, (scrapwood)
wrote:
I have a question that I can't seem to find an answer to.

The task is adding a new sub-panel in an outbuilding. *I understand that
new sub-panels now require a 4-wire feeder line, ...

Run four. It never hurts to upgrade and it is always far more
bother and expense to change it later when the need, that you thought
would never happen, happens.